U.S. Pat. No. 1,805,933 PA1 U.S. Pat. No. 2,487,496 PA1 U.S. Pat. No. 2,841,894 PA1 U.S. Pat. No. 2,999,697 PA1 U.S. Pat. No. 3,302,958 PA1 U.S. Pat. No. 3,310,330 PA1 U.S. Pat. No. 3,378,084 PA1 U.S. Pat. No. 3,431,005 PA1 U.S. Pat. No. 3,563,595 PA1 U.S. Pat. No. 3,608,942 PA1 U.S. Pat. No. 3,987,562 PA1 Australian Letters Pat. No. 278,004